ct530815

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Vermelding onderdeel organisatie February 1, 2012 1 Chapter 15: Failure modes and optimisation ct5308 Breakwaters and Closure Dams H.J. Verhagen Faculty of Civil Engineering and Geosciences Section Hydraulic Engineering Sri Lanka, Kudawella Tsunami damage of breakwater 2004

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Page 1: Ct530815

Vermelding onderdeel organisatie

February 1, 2012

1

Chapter 15: Failure modes and optimisation

ct5308 Breakwaters and Closure Dams

H.J. Verhagen

Faculty of Civil Engineering and Geosciences Section Hydraulic Engineering

Sri Lanka, Kudawella Tsunami damage of breakwater

2004

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February 1, 2012 2

What is the most important element of a breakwater or closure dam ??

• the element which is the most expensive one

• the section which is the most costly one

• the element which is the most unreliable one

• the element which is the most sensitive to variations in the boundary conditions

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failure modes for dike-type structures

Failure of breakwater by earthquake

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February 1, 2012 4

failure modes for a rubble mound (Burcharth, 1992)

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Failure modes for a monolithic breakwater

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February 1, 2012 6

rock fill overflow dam failure modes

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February 1, 2012 7

fault tree

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fault tree for closure dam (cross section)

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February 1, 2012 9

fault tree for closure dam (equipment)

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February 1, 2012 10

fault tree for construction planning

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equipment utilisation in relation to fault tree

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The Dilemma

• A strong and heavy breakwater does not require maintenance … but is very expensive to construct

• A light breakwater is much cheaper to construct … but requires a lot of maintenance

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Wave climate

Wave Height H

(m)

Probability of

Exceedance

(times per annum)

4 1.11

5 1.58*10-1

5.2 8.4*10-2

5.5 7.62*10-2

5.8 3.8*10-2

6 2.47*10-2

6.5 7.35*10-3

7.15 3.0*10-3

7.25 2.63*10-3

7.8 9.0*10-4

7.98 8.0*10-4

8.7 1.5*10-4

wave exceedance

0

2

4

6

8

10

0.00010.0010.010.11

exceedance (times per year)

wa

ve

he

igh

t (m

)

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February 1, 2012 14

development of damage

Actual Wave Height H Damage in % of armour layer

H < Hnd 0

Hnd < H < 1.3Hnd 4

1.3Hnd < H < 1.45 Hnd 8

H > 1,45 Hnd Collapse

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February 1, 2012 15

Cost of construction

Design wave height

Hnd

Initial cost

breakwater

“C”

Initial cost Amour

Layer

“A”

(m) (€) per running meter (€) per running meter

4 13900 5280

5 15220 6600

5.5 15900 7280

6 16540 7920

Initial cost for armour units: € 1320 * Hd

for core: € 8620

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February 1, 2012 16

annual risk

1 < H < 1.3 Hnd

n = 4% damage

1.3 Hnd < H < 1.45 Hnd

n = 8% damage

H > 1.45 Hnd

Collapse

Hnd

p w p.w p w p.w p w p.w

(m) (1/year) (€) (€/year) (1/year) (€) (€/year) (1/year) (€) (€/year)

4 1.02 420 430 4.6 10-2

860 40 3.8 10-2

13900 530

5 1.5 10-1

530 80 4.7 10-3

1060 5 2.6 10-3

15220 40

5.5 7.4 10-2

580 40 2.2 10-3

1160 - 8 10-4

15900 10

6 2.4 10-2

630 15 7.5 10-4

1260 - 1.5 10-4

16540 3

p probability of occurrence of the wave height

w cost of repair of the armour layer (2nA)

respectively cost of replacement (C)

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February 1, 2012 17

average annual risk

s = (p.w) Hnd

Full repair of partial

damage

Only repair of serious

damage(>8%)

No repair of partial

damage

(m) (€ per year) (€ per year) (€ per year)

4 1000 570 530

5 125 45 40

5.5 50 10 10

6 18 3 3

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capitalised maintenance cost

Capitalised risk S Hnd

Full repair of partial

damage

Only repair of serious

damage(>8%)

No repair of partial

damage

(m) (€) (€) (€)

4 30000 17100 15900

5 3750 1350 1200

5.5 1500 300 300

6 540 90 90

lifetime of 100 years; rate of interest 3.33%

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February 1, 2012 19

total cost

Total cost I + S Hnd

Full repair of partial

damage

Only repair of serious

damage(>8%)

No repair of partial

damage

(m) (€) (€) (€)

4 43900 31000 29800

5 18970 16570 16420

5.5 17400 16200 16200

6 17080 16630 16630

6.5 17300

Adding up initial cost plus capitalised maintenance cost

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total cost for various strategies

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

3.5 4 4.5 5 5.5 6 6.5

H-design

co

st

full repair

partly repair

no repair

initial cost

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Conclusions for Rubble Mound Breakwaters

• There is an optimum design wave height

• Accepting regular maintenance is the best option

• This implies that the design also should allow a “repairable” breakwater

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differences in breakwater type

• In case of overload a Rubble mound breakwater will suffer from some damage, which can be repaired.

• In general for Rubble mounds:

the amount of repair costs increase linear with the amount of overload: cost = B * (Hstorm - Hdesign)

• In general for Vertical wall breakwaters:

you have always a given fixed amount of damage, not depending on the amount of overload: cost = A + B (Hstorm - Hdesign)

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Vertical wall breakwater

0

50000

100000

150000

200000

250000

300000

350000

400000

5 7 9 11

Hs (design wave)

tota

l co

st

per

mete

r

building cost

yearly repair

capitalised repair

total

damage cost = 4000 + 1500*Hs

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Conclusions

0

10000

20000

30000

40000

50000

5 7 9 11

Hs (design wave)

tota

l co

st

per

mete

r

0

10000

20000

30000

40000

50000

5 7 9 11

Hs (design wave)

tota

l co

st

per

mete

r

Rubble mound breakwater Vertical wall breakwater

total

repair

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February 1, 2012 25

Conclusions (2)

• Rubble mound breakwaters are less sensitive to uncertainties in wave data

• If there is no overload, a Vertical wall breakwater requires less maintenance

• If there is overload, Vertical wall breakwaters cause much more problems

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Including “secondary damage”

• When a breakwater is damaged, the port cannot function well

• The cost because of loss of production should be included in the calculation

• In general secondary damage will not change the tendency of the conclusion before, but make them more even more stronger: • The optimum for a rubble

mound breakwater is allowing quite some damage, and doing a lot of repair